This is an interesting article about McDonald's food production.
The chemical in question, tertiary Butylhydroquione, is a petroleum-based product that can cause health effects such as stomach tumors in lab animals when administered in high doses. The compound, used in McDonald's Chicken McNuggets, meets Chinese food safety standards. However, Liu Qingchung, a nutritionist at the General Hospital of Armed Police Forces, was quoted in China Daily as stating that the chemical used in McDonald's chicken nuggets is "toxic to some extent."
The additive is also used in McDonald's restaurants in the U.S. and Japan, where it also meets national health standards.